Pain in my ginocchio

I've had tons and tons of times I thought I was injured in the last 10+ years of running.  Usually random pains that go away after a few days off, usually coinciding with peak weeks or taper time.  I've also had a few falls, resulting in a couple sprained ankles.  Plus I've had two legit injuries -- a pulled muscle back in 2005, and then I broke my ankle in 2015 when I stepped on a rock.  Those are the only two times I've been off running for over a month. 


Oddly enough, in all this time running, I've never had real knee issues that I can remember.  It always seems to be my feet, ankles and calves that get hurt. 


But suddenly, knee pain! 


Oh yeah, the Italian word for knee is "ginocchio."  (Pronounced -- jin -OH -kee -oh, which is odd, because occhio means "eye.") 


Well, somehow my left knee is about the size of a softball right now.


Here's the chronology:


Sunday:  half marathon, pretty slow.  Well, 5-7 miles as planned, then a very slow slog to the finish.  It got into the upper 80s that day, super sunny, not fun.  No pains. 


Monday:  boot camp, less than one mile of running included.  Quads somewhat sore for no clear reason. 


Tuesday:  5 miles, mostly about my average pace from Sunday (so I'd say medium effort).  No pain.


Wednesday:  boot camp, including probably 1 mile of running, and then about 28 flights of stairs up and then down (Mount O) (not something we do often, but probably once a month?).  No pain.  Came home from boot camp, had breakfast, then ran to work.  No pain.  Walking through the lobby and noticed my left knee hurt.  Like hurt.  Went to my desk, went down the stairs to shower (8 flights).  Showered, took the stairs back to my office.  By then my knee really hurt.  It was popping at times, especially at angles, and it felt pretty swollen.  Almost screaming pain when I tried to move my leg certain ways while seated at my desk (so no pressure on my foot).  Wasn't sure how I was going to run home.  Considered getting an uber, but decided to set out after work.  Did a very slow run-walk thing to get home, no pain while running or walking.


Thursday:  5 miles, again, no pain while running.  But my knee was so swollen other people could tell something was horribly wrong.  Again, crazy pain when bending it at certain angles, very painful pops.


Friday:  rest day.  Maybe getting less swollen and painful?  Laying on my back, trying to bend my leg all the way and get my heel toward my butt, I feel like I can't get within 12 inches on the left side, my knee hurts and is so swollen. 


Planning to rest today, and maybe skip my planned "long" run for tomorrow (only 7 miles since there's no race on the calendar now). 


What the heck? 


Part of me thinks it could just be sore from the stairs since yesterday I seemed to feel pain mostly when going up stairs at home.  Maybe it will blow over with a weekend off?  I really don't want it to be anything real.